Apparatus for detecting changes in the thickness of an ink film on the roller system of a printing press

ABSTRACT

Apparatus for detecting the thickness of an ink film on the roller of a printing press either to indicate the film thickness and/or to control one or more ink supplies to the roller. A monitoring head includes a housing for a transparent roller which, in use, contacts a printing press roller. The housing contains a light source positioned between two photoconductive cells, one in the transparent roller and one fixed in the housing. The photoconductive cells have a selective response. Either optical filters are positioned before the photoconductive cells or a monochromatic light source is employed. The photoconductive cells are included in a bridge circuit which has variable resistors for controlling its output and sensitivity. Accurate correlation is possible between ink film thickness and the output of the bridge circuit. Systems are disclosed using multiple monitoring heads and servo-controls for controlling ink supply to single color and multicolor presses.

United States Patent Heasman et al.

[451 Dec. 26, 1972 [72] Inventors: David Michael Heasman, Dorking Surrey; Andrew Wallace Little, Whitton, both of England [73] Assignees: The Research Association For the Paper and Board Printing and Packaging Industries, Surrey; Churchill Instrument Company Limited, Middlesex, England [22] Filed: Nov. 5, 1970 [21] Appl. No.: 87,085

[30] Foreign Application Priority Data Nov. 14, 1969 Great Britain ..55,892/69 [52] US. Cl. ..10l/207, 101/350, 101/365, 250/210 [51] Int. Cl. ..B4lf 31/04, H0lj 39/12 [58] Field of Search ..l0l/335, 349, 350, 365, 366, 101/202, 206, 207, 208; 250/210, 219, 226; 356/195, 173, 199

[56] References Cited UNITED STATES PATENTS 3,336,481 8/1967 Nelson ..250/226 3,604,815 9/1971 Clemens ..250/210 3,353,484 11/1967 Koyak ..l01/350 2,510,347 6/1950 Perkins... ...250/2l0 3,535,046 10/1970 Denner ...250/210 X 3,231,745 l/l966 Doubek, Jr. et ..250/2l0 3,535,528 10/1970 Strickland ..250/210 FORElGN PATENTS OR APPLICATIONS 1,140,107 l/1969 Great Britain ..10l/335 Primary Examiner-J. Reed Fisher Attorney-Baldwin, Wight & Brown 57 ABSTRACT Apparatus for detecting the thickness of an ink film on the roller of a printing press either to indicate the film thickness and/or to control one or more ink supplies to the roller. A monitoring head includes a housing for a transparent roller which, in use, contacts a printing press roller. The housing contains a light source positioned between two photoconductive cells, one in the transparent roller and one fixed in the housing. The photoconductive cells have a selective response. Either optical filters are positioned before the photoconductive cells or a monochromatic light source is employed. The photoconductive cells are included in a bridge circuit which has variable resistors for controlling its output and sensitivity. Accurate correlation is possible between ink film thickness and the output of the bridge circuit. Systems are disclosed using multiple monitoring heads and servo-controls for controlling ink supply to single color and multicolor presses.

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PATENTED DEC 26 I972 SHEET 7 [IF 7 WMJMM APPARATUS FOR DETECTING CHANGES IN THE THICKNESS OF AN INK FILM ON THE ROLLER SYSTEM OF A PRINTING PRESS CROSS REFERENCES TO RELATED APPLICATIONS Issued British Pat. No. 1,140,107 assigned to The Printing, Packaging and Allied Trade Research Association relates to apparatus'for detecting changes in the thickness of an ink film on the roller system of a 1 printing press.

BACKGROUND OF INVENTION 1 Field This invention relates to apparatus for detecting changes in ink film thickness on a printing press roller and more particularly to an improvement of such apparatus that employs a pair of photocells, one contained in a transparent roller in contact with a printing press roller to generate a film thickness signal, the other being provided to generate a reference signal.

2. Description of prior art 7 In British Pat. No. 1,140,107 Heasman and Adams describe the latter mentioned apparatus in which the transparent roller picks up an ink film which reduces the amount of light passing from a lamp to the photocell via the ink film, hence providing a signal which varies with variation in ink film thickness. A reference cell is provided which is also illuminated by the lamp, the output signal of which cell is controlled by a mechanically adjusted diaphragm or shutter. Both photocells are connected in a bridge circuit, the output and sensitivity of which circuit may be adjusted by single respective variable resistances. The intensity of illumination of the reference photocell can be separately controlled by a variable resistor in series with the lamp but not forming part of the bridge circuit.

It is the principal object of the present invention to improve the apparatus of the above described prior art to provide a more stable output which is also accurately proportional to print density and hence film thickness; the improved apparatus being finely adjustable and settable to such accuracy as to allow correlation between different presses such as a proofing press and a production printing press, correlation between different points in a roller train of the same press, and correlation between color and print density.

It is a further object of the invention to provide a detection head and a signal responsive circuit whereby similar monitoring heads may be interchanged for connection to the same responsive circuit without the need for recalibration, or whereby a plurality of heads can be manually or automatically monitored to adjust manually or automatically individual ink supplies to a press, or whereby a plurality of heads can be combined with the said signal responsive circuit.

It is another object of the invention to provide improved apparatus with such a linear and proportioned response that an indicating means, connected in a bridge circuit served by a detection head, can be calibrated uniformly in readings related to print density or provide a linearly related digital output.

It is a further object of the invention to provide such a constantly related print density/output signal that a warning device can safely be set to warn a printer of variants during production from a predetermined range of print densities.

SUMMARY OF INVENTION 0 The output of the bridge circuit is thus electrically controlled. A first or detection photoconductive cell and a second or reference photoconductive cell with a peak response between 0.5 micron 0.7 micron are provided with blue optical filters, the spectral response of said filters being flat within the range 0.5 micron 0.7 micronfAltern atively, a plurality of monochromatic light sources and a plurality of photoconductive cells of matching spectral response are provided selectively to monitor ink of corresponding colors. The bridge output is thereby directly proportional to the intensity of light falling on said first photoconductive cell after passing through an ink film on said transparent roller. Hence the bridge output is directly proportional to changes in the thickness of said ink film and hence print density.

When used, the filter positioned before the first or detection photoconductive cell is movable either to cover or uncover said cell. By covering said cell with the filter, a linear response to ink of one or more colors is obtainable, by uncovering said cell, response to black ink is increased.

The sensitivity of the bridge circuit is adjustable by variable impedance means in series with its output leads whereby sensitivity of the improved apparatus can be compensated for differently colored ink films. This variable impedance may be a stepped resistor to compensate for differently colored inks, sensitivity being adjusted simply by turning a switch to select a resistance appropriate to the color.

The first and second photoconductive cells, optical filters, light source and transparent roller are mounted in a housing to form a monitoring head. One or more of such heads may be sequentially connected to a bridge circuit. Also a plurality of heads connected to respective bridge circuits can be employed in a system for monitoring either a single color or a multicolor press. In the latter system, servo-control systems are connected to each bridge circuit for controlling ink supply means for the printing press.

DESCRIPTION OF DRAWINGS Other features, objects and advantages of the invention will be apparent from the description hereinbelow taken in conjunction with the following drawings in which:

FIG. 1 illustrates a film thickness monitoring head according to the present invention;

FIG. 2 illustrates a bridge circuit according to the present invention for the monitoring head shown in FIG. 1;

FIG. 3,shows alternative energization of the bridge circuit and lamp illustrated in FIG. 2;

FIG. 4A and 4B show typical linear meter responses plotted against print density for various colored inks, the graphical representation being obtained from tests with apparatus according to the present invention;

FIG. 5 illustrates a multiple monitoring head system for a four color printing press, according to the present invention;

FIG. 6 illustrates the press rollers, ink supplies and control means of the system shown in FIG. 5;

FIG. 7 illustrates adjustment means for an ink supply duct of each press used in the embodiment described with reference to FIGS. 5 and 6;

FIG. 8 illustrates the inking roller, ink supplies and control means for multiple monitoring head system for a single color press, according to the present invention; and

FIG. 9 illustrates adjustment means for an ink supply duct of the press in the embodiment described with reference to FIG. 8.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, a transparent tubular roller 1 is supported, free to rotate about a pivot (not shown) mounted relative a casing 11. Casing 11 is fixed to the printing press by a pivot (not shown) situated at the rear of the fixed photocell 19. The arms 3 are supported at a pivot 6 co-axial with a bearing for a transfer roller 7 pivoted at 5. Two cranked arms 3 must be provided, one on each side-of the casing 11 to support the transfer roller 7 when contact is made by the monitoring head of FIG. 1 with a metal roller or an oscillating roller 9. The transparent roller 1 makes contact with, and is rotated by the transfer roller 7, the transfer roller 7 transferring an ink film from the roller 9 which forms part of a printing press inking system.

The transparent roller 1 houses a photoconductive cell 16 fixed on its axis of rotation. The casing 11 contains white light source or a lamp [7 and the fixed photoconductive cell 19 arranged to receive light through an optical filter 19' directly from the lamp 17. An aperture 2 is'provided between the lamp l7 and the transparent roller 1 to allow light from the lamp 17 to pass thereth rough after passing through an optical filter l6 and the ink film adhering to the transparent roller 1, the cylindrical wall of the transparent roller 1, and finally to the cell 16. Screens 4 are provided around the transparent roller 1 to ensure that substantially only the light reaching the cells 16 and 19 comes from the lamp l7 and, in the case of the cell 16, passes through the ink film on the roller 1. Optical filters 16' and 19 are blue filters with a substantially flat spectral response between 0.5 micron 0.7 micron, which blue filters may, for example, be the 088 filter of Chance-Pilkington the spectral transmission which can be found on page 13 of the March, I969, edition of Color Filter Glass. Photoconductive cells 16 and 19 have a peak spectral response between 0.5 micron 0.7 micron and are, for example, cadmium sulphide cells or photoconductive cell RPY 4l obtainable from Mullard the spectral response of which can be seen from page C1 of their August I966 handbook.

The filter 16 is preferably movable either to cover or uncover cell 16. The filter 16' can be manually moved by a filter selector switch (not shown) coupled to shaft 50 about which the filter 16' is hinged. Reference numeral 16" identifies filter 16' in a position uncovering the cell 16.

In use, the monitoring head shown in FIG. 1 is lowered onto the roller 9 of a printing press whereby the transfer roller 7 transfers ink from roller 9 to the transparent roller 1. The film of ink picked up by the transparent roller 1 is related to the thickness of the ink film on the printing roller 9. The intensity of light reaching cell 16 depends on the thickness of the ink film on the roller 1. It will of course, also depend on the intensity of illumination of the lamp 17 but this, for a given set of conditions, is held as constant as is practical. The intensity of light reaching the cell 19 depends only on the intensity of illumination of the lamp 17 (it being understood that both cells 16 and 19 will respond to the spectral cut-off due to their inherent characteristics and the filters 16 and 19'). In the previous invention of Heasman and Adams disclosed in British Pat. No. I,l40,l07, the amount of light reaching photocell 19 was controlled by a mechanical diaphragm or shutter interposed between photocell l9 and lamp 17. However, according to this invention, the previous mechanical diaphragm is dispensed with in favor of controlling electrically the output of a bridge circuit to which' both cells 16 and 19 are connected. This is explained in detail below.

Referring to FIG. 2, this schematically illustrates a bridge circuit for use with the head described with reference to FIG. 1'. A bridge circuit generally shown at 23 includes the two cells 16 and 19 in series connected arms. The other two series connected arms of the bridge contain a fixed resistor 27 in one arm, and two variable resistors 25 and 29 in another arm. These two latter series connected arms are connected through the resistance track of a variable resistor 31 the slider of which is connected to the bridge output. Variable resistor 29 provides fine adjustment of the bridge output.

The bridge output is shown connected to an electrical meter 37 which may be, for example, a center zero pointer microammeter, which meter is connected in parallel with a potentiometric recorder 41 and a capacitor 39.

The capacitor 39 sets the damping of the meter 37 so that its response time is longer than the time of one revolution of the transparent roller 1. By this means, rapid fluctuations in the indications of meter 37 due to marks or striations on the roller 1 are prevented. The potentiometric recorder 41 provides a record of variations in the state of the bridge with time. Connected in series with the parallel circuit of the recorder 41, the meter 37 and the capacitor 39, is another variable resistance 42 for adjusting the sensitivity of the bridge for example, for color compensation.

The circuit illustrated in FIG. 2 also includes a rectifying circuit 47 for providing DC to energize the bridge 23. The lamp 17 is energized by a voltage stabilizer 43 which also supplies the rectifying circuit 47, the stabilizer 43 being connected in series with AC mains. Alternatively the circuit illustrated in FIG. 3 includes a rectifying circuit 43 for providing DC for a voltage stabilizer 47 which provides a stable voltage for energizing the bridge 23. The lamp 17 is fed by a low voltage AC source 48.

Variable resistor 25 is adjusted to control the output of the bridge 23 in relation to the amount of light received by cell 16 from the lamp 17. This', as previously mentioned, replaces the movable diaphragm before cell 19 used in the prior art. However, it is also important to note that to change the intensity of illumination of lamp 17 according to the above described prior art, the voltage of the supply to the lamp was altered by a variable resistor in series therewith. Coarse balance control of the bridge circuit was effected by the movable diaphragm and color compensation or variation in the sensitivity of the monitor to different colors was effected by moving the diaphragm and controlling the variable resistor in series with the lamp 17.

However, changing the intensity of light from the lamp 17 by varying its voltage naturally changed the spectrum of emitted light (as it becomes dull the blue content decreases). Furthermore, adjustment of the movable diaphragm was only a coarse control and in practise unreliable for calibration to effect color compensation. The amount of light passing through the adjustable diaphragm was not measureable.

Thus the main drawback and problem attached to use of the prior art apparatus was the very great practical difficulty of allowing a simple adjustment to be made to compensate for color changes or other changes in operational conditions such as replacing the monitoring head.

It must be explained that under normal circumstances an optical examination is made during printing to determine if the print density is acceptable. Reflection densitometers are available for such measurements but their use means delay in assessing print quality as measurement can only be made after paper has been printed. Different illuminations of a surface will give different reflections so clearly estimation of print density by eye cannot always be relied on. The spreading of ink on a surface of a small roller produces the effect of a light filter i.e., different densities are given according to the thickness of the film adhering to the rollers, and the density is related to the color of ink which is used.

Thus, correlation is required between light transmitted to the detector photocell l6 and the print density (normally determined by reflection measurement) of a print resulting from production. It is not possible to use the entire spectral response of the human eye since this results in non-linearity, and a monochromatic light source, although giving the easiest control, would be limited to using ink of one color. However, it is possible to use a range of monochromatic light sources, each with a corresponding narrow band or line spectral output, and to provide corresponding range of compatible photoconductive cells, each monochromatic light source and cell being selected for use for a corresponding ink. The color of each light source is complementary to the color of the ink illuminated thereby.

It was discovered that colors outside the spectral range of 0.5 micron 0.7 micron effect the response of, for example, a cadmium sulphide cell by a maximum of only percent of its output. In other words, the response of such a cell peaks in the 0.5 0.7 micron region. To modify this response, blue filters l6 and 19 also having a substantially flat transmission in the 0.5 0.7 micron waveband are mounted before the photoconductive cells 16 and 19 respectively. Transmitted light within the selected waveband response changes the resistance of a cell 16 in direct proportion to the intensity of light transmitted through the ink film adhering to the transparent roller 1. This feature combined with the use of variable resistor (replacing the movable diaphragm) enables accurate calibration and recalibration and furthermore, the ability to insert stepped resistors in place of variable resistor 42, which may be switched to change from one color to another without requiring a change in components of the bridge circuit. Thus, a comparison of print density can be made irrespective of color, for example 10 meter divisions on red ink corresponds to 10 meter divisions on yellow ink and this correlation has been checked with various colors by a reflection densitometer the results being graphically illustrated in FIGS. 4A and 48. FIG. 4A shows the sensitivities of each color without resistor 42 in the bridge output and it can be seen that different colors have different sensitivities, i.e. the slope of each curve is different. However, with the addition of resistor 42, the sensitivities can be matched as shown by FIG. 4B to allow correlation between colors.

It will be appreciated by those skilled in the art that:

Print density =1og1o In practise a print density difference is read from indicating means. For example, the press is set up in the usual way by the operator until the printed product is satisfactory by reference to a pass sheet or when checked by densitometer. At this stage the operator selects the appropriate setting for the color of the ink, and the type of paper (coated or uncoated) in use by turning a multi-point switch connected to stepped resistor 42 on control unit. He then sets the indicating meter, for example, a center zero-ammeter, to center zero. I

Changes in ink film thickness deflect the meter and indicate to the operator when and in which direction adjustment of the ink duct setting is required to maintain the consistency of the end product. Changes due to ink viscosity color, duct loading, temperature and stand-off of ink from the duct roller will be indicated, permitting corrective action to be taken before any detectable change occurs in the print.

Sensitivity to different colors is present to suit most applications, but can be adjusted according to requirements. The initial setting of the monitor can be recorded so that the same results can be obtained at different times, and results obtained from different presses can be correlated.

If the bridge 23 is energized with stabilized fixed frequency AC, a suitable detector replacing recorder 41, meter 37 and capacitor 39 can be used with variable inductors or capacitors in place of variable resistors 25 and 42.

Referring to FIGS. 5, 6 and 7, a multiple monitoring head system for use on a four color press will now be described. Four monitoring heads complete with bridge circuits 51 54 are used to monitor the inking system 55 58 of a four color press. The transparent rollers 51' 54' of each head 51 54 are shown in contact with corresponding rollers in the roller train of each press. Ink of respective colors is supplied to each roller train from ducts 59 62.

The output of the bridge circuit of each monitoring head is connected in series with respect stepped resistors 63 66. The sliding contacts 67 70 of each stepped resistor 63 66 is respectively connected to contacts 71 74 of a rotary switch 75. The sliding contact 76 of switch 75 is connected via indicating means 77 to the bridge outputs to complete respective output circuits. Contacts 71 74 of switch 75 are also connected to respective servo amplifier systems 78 81. The output of each servo amplifier systems 78 81 is connected to respective adjustment means to control ink flow from each of the ducts or fountains 59 62.

FIG. 7 illustrates a typical adjustment means in a known ink supply arrangement where an electric motor 82 drives, by means of a belt 83, a wheel 84 which controls the rotation of a duct or fountain roller 85 of the duct or fountain 59. The control system in its entirety is shown more clearly in FIG. 6 relative the inking systems of the four color press.

The servo amplifier systems can be known systems on based on such, the requirement being to amplify the bridge output or. outputs, preferably with error correction and/or negative feedback stabilization, and to provide a stable output signal to control adjustment means for each duct or fountain 59 62. The ink supply to the press can be of a known type, for example where the duct or fountain resembles a V-shaped trough (FIG. 7) formed by a metal duct (fountain) roller 85 and a flexible steel blade 100. Ink is drawn from the duct or fountain by downward rotation of the roller 85 and is transferred to the ink distribution system of the press by a vibrator (ductor) roller 101. The axis of rotation of the vibrator roller 101 swings between two positions, one making contact with the duct (fountain) roller 85 and the other making contact with a roller 102 in the ink distribution system.

The amount of ink transferred from the duct (fountain) is controlled in two ways.

i. The required ink distribution across the press, which is determined by the printed image distribution (e.g. from unprinted paper through type to large colored areas), is controlled by keys or screws 90 spaced evenly across the duct (fountain). These keys or screws 90 can be adjusted to vary the pressure between blade 100 and roller 85 so as to vary the thickness of ink film drawn at different points from the bottom of the duct (fountain). Such adjustments are primarily made by the press operator at the start of a printing run.

ii. The amount of this variable ink film that is transferred to the ink distribution system can be varied overall by increasing or decreasing the angle through which the duct (fountain) roller 85 rotates during the time it is in contact with the vibrator (ductor) roller 101.

In use, the sliders 67 70 of each stepped resistor 63 66 is connected to a resistance to compensate for the color monitored by its associated monitoring head 51 54. When so adjusted, indicating means 77 connected to a respective stepped'resistor 63- 66, will indicate-a reading related to an arbitary scale of print density difference measurements.

The ink supplies from ducts 59 62 are continuously and automatically monitored by the servo amplifier systems 78 81 which each respond to the outputs of the corresponding bridge circuits. The system illustrated may for example, be supplied with yellow, magenta, cyan and black ink.

The indicating means, bridge circuits and servo amplifier systems may be contained in a single control unit structure as identified by block 86 in FIG. 6. The rotary switch may be either manual or automatic to give selected indications on the indicating means 77 of the print density of each color, (FIG. 5).

Referring now to FIGS. 8 and 9, a description will now be given of a multiple monitoring head system for use on a single color press. The circuitry schematically shown in FIG. 5 is used in this system, the only exception being that each sliding contact 67 70 of the stepped resistors 63 66 is turned to select the same resistance in view of using ink of one color. Referring to FIG. 8, the monitoring heads 51 54 are mounted with their transparent rollers 51' 54' in contact with an inking roller 87. Lengthwise of the inking roller 87 are a plurality of ink ducts controlled by a plurality of duct keys 9.0. Control of each duct key 90 isbest seen from FIGS. 7 and 9 where, in FIG. 7, a DC stepping motor 89 is shown coupled to a duct key 90.

Indicating means, bridge circuitry, and servo amplifier systems are contained in block 91 identified in FIG. 8.

Operation of the multiple monitoring head system for the single color press is similar to the operation for the four color press so no further description will be given.

Preset inking levels can be set on the control units 86 and 91 of the above described systems whereby precise control of ink supply is effected.

The systems disclosed and described with reference to FIGS. 6 and 8 employ monitoring heads provided with photoconductive cells 16 and 19 havinga peak response between 0.5 0.7 micron, blue filters with a flat response in the same waveband and a white light source.

In conclusion, the improvement of the present invention over the prior art enables the following to be accomplished:

a. Monitoring heads can be interchanged between bridge circuits. 7

b. Multiple heads can be combined in a construction using a single bridge circuit.

0. Multiple heads can be monitored either manually or automatically to enable individual duct keys of a press to be manually or automatically adjusted respectively.

. digital readout of print density difference can be printed or displayed.

e. Correlation is possible between print density readings on a proofing press and a production printing press.

f. The stability of the apparatus is enhanced to the point where information can be obtained by placing the same monitoring head at different points in the roller train of a press, and comparing the signals obtained. For example, faults such as outof-round rollers, damaged rollers, and faulty roller bearings can be detected in a roller train where they effect ink film thickness.

g. By providing a truly proportional output to print density, the acceptable limits for a print will bear a constant relationship with the electrical output of the bridge circuit. This enables alarms or warning devices to be set so that the attention of a printer is drawn to variance that fall outside a predetermined acceptable range.

Changes of variations in the above described embodiment without departing from the scope of the invention may be readily apparent to those skilled in the art. The scope of the invention is defined by the claims appended herewith.

What is claimed is:

1. In apparatus for detecting changes in the thickness of an ink film on a roller system ofa printing press comprising a transparent roller for rotation with a roller of said press to pick up an ink film therefrom, a first photoconductive cell disposed within the transparent roller, a light source arranged to illuminate the first photoconductive cell through the surface of the transparent roller, and a second photoconductive cell arranged to receive from the light source light whose intensity varies, in use, only as the intensity of the light source varies, and a bridge circuit including said photoconductive cells, the improvement comprising a variable impedance in one arm of said bridge circuit to adjust the output thereof according to the intensity of light received by said first photoconductive cell, and optical filter means positioned before each of said photoconductive cells, said first and second photoconductive cells having a substantially peak spectral response between 0.5 micron 0.7 micron and said filter means having a substantially flat spectral response between 0.5 micron 0.7 micron.

2. The invention according to claim 1 wherein said variable impedance is a variable resistor.

3. The invention according to claim 2 in which said variable resistor is in series with another variable resistor which other variable resistor provides fine adjustment of said bridge output.

4. The invention according to claim 1 wherein said bridge output is connected to electrically operated indicating means calibrated with a linear scale related to print density.

5. The invention according to claim 1 wherein said bridge output is connected to electrically operated indicating means providing a digital output related to print density.

6. Apparatus for detecting changes in the thickness of an ink film on a roller system ofa printing press comprising in combination:

a. A monitoring head including first photoconductive cell means; a transparent roller containing said first photoconductive cell means and provided for rotation with a roller of said printing press; second photoconductive cell means; said first and second photoconductive cell means having a substantially peak response of 0.5 micron 0.7 micron; an electrical light source disposed between said first and second photoconductive cell means (and capable of emitting light of substantially all colors); first and second optical filter means with a substantially flat spectral response of from 0.5 micron 0.7 micron and disposed before said first and second photoconductive cell means respectively; and housing means for housing said transparent roller, said second photoconductive cell means, said optical filter means, and said electrical light source;

b. A bridge circuit including said first and second photoconductive cell means in first and second arms thereof; impedan c means in respective third and fourth arms thereof; said impedance means in said third arm including two variable resistors, one for fine adjustment of the output of said bridge circuit, and the other for adjustment of said bridge output according to the light received by said first photoconductive cell means; responsive means connected to said bridge output; and terminals for connection to energization means for energizing said bridge circuit.

7. Apparatus according to claim 6 wherein said responsive means is an indicator calibrated with a linear scale related to print density.

8. Apparatus according to claim 7 wherein said indicator is provided with a digital read-out.

9. Apparatus according to claim 6 wherein said responsive means is a servo-control system connected to said bridge output for controlling the supply ofink to said printing press.

10. Apparatus according to claim 9 wherein a plurality of said monitoring heads each connected to respective ones of a plurality of said bridge circuits are provided to monitor differently colored ink films on the rollers of a color printing press, each respectively monitored roller being supplied with ink from controllable supply means, and wherein said variable resistor for varying the sensitivity of said bridge circuit is a stepped resistor for compensating said bridge output for different colored ink films, individual ones of said ink supply means being controllable by corresponding outputs from said bridge circuits.

11. Apparatus as claimed in claim 9 wherein a plurality of said monitoring heads each connected to respective ones of a plurality of said bridge circuits, the monitoring heads being provided to monitor ink film thicknesses detected at different points across the length of a duct or fountain roller on the inking system of a printing press, said duct or fountain roller being supplied with ink from a duct or fountain including a plurality of adjustable ink duct or fountain keys, individual ones of said keys being controllable by corresponding outputs from said bridge circuits.

12. Apparatus according to claim 9 including a plurality of said monitoring heads, said monitoring heads being selectively individually connectable to said bridge circuit.

13. Apparatus according to claim 6 wherein said responsive means includes a warning device for warning a printer when the print density of prints produced by said printing press falls outside predetermined limits.

14. An improvement in, or modification of, apparatus for detecting changes in the thickness of an ink film on a roller system of a printing press, said apparatus comprising a transparent roller capable of rotating in contact with one of the rollers of the press inking system to pick up an ink film therefrom; a first photocell disposed within said transparent roller; a light source arranged to illuminate said first photocell through the surface of said transparent roller; and a second photocell arranged to receive from said light source, light whose intensity varies, in use, only as the intensity of said light source varies; means for deriving a signal from the output of said first and second photocells in combination, said signal being selected from the type to provide a measure of the thickness of said ink film adhering to said transparent roller and the type to control the ink flow to said roller system of the printing press, said first and second photocells being so connected in a bridge circuit that, in use, the output of said bridge circuit will vary only to the light transmitted through said transparent roller to said first photocell, the improvement comprising a variable impedance connected in one of the arms of said bridge circuit for adjustment to vary the bridge output in relation to the amount of light received by said second photocell, and a variable impedance connected in series with the output of said bridge circuit for adjusting the sensitivity thereof.

15. A monitoring head for apparatus for detecting changes in the thickness of an ink film on a roller system of a printing'press comprising first photoconductive cell means; a transparent roller containing said first photoconductive cell means and provided for rotation with a roller of said printing press; second photoconductive cell means; said first and second photoconductive cell means having a substantially peak spectral response of between 0.5 micron 0.7 micron; an electrical light source disposed between said first and second photoconductive cell means; first and second optical filter means with a substantially flat spectral response of from 0.5 micron 0.7 micron disposed before said first and second photoconductive cell means respectively; and housing means for housing said transparent roller, said second photoconductive means, said optical filter means and said electrical light source,

16. Apparatus for detecting changes in the thickness of an ink film on a roller system ofa printing press comprising in combination:

a. A monitoring head including first photoconductive cell means; a transparent roller containing said first photoconductive cell means and provided for rotation with a roller of said printing press; second photoconductive cell means; an electrical monochromatic light source disposed between said first and second photoconductive cell means and capable of emitting light of a color complementary to the color of the ink the thickness of which is to be detected, said first and second photoconductive cell means having a spectral response to the light from said monochromatic light source; and housing means for housing said transparent roller, said second photoconductive cell means and said monochromatic light source;

b. A bridge circuit including said first and second photoconductive cell means in first and second arms thereof; impedance means in respective third and fourth arms thereof; said impedance means in said third arm including two variable resistors, one for fine adjustment of the output of said bridge circuit, and the other for adjustment of said bridge output according to the light received by said first photoconductive cell means; responsive means connected to said bridge output, variable resistive means connected in series with said responsive means for adjusting the sensitivity of said bridge circuit; and terminals for connection to energization means for energizing said bridge circuit. 

1. In apparatus for detecting changes in the thickness of an ink film on a roller system of a printing press comprising a transparent roller for rotation with a roller of said press to pick up an ink film therefrom, a first photoconductive cell disposed within the transparent roller, a light source arranged to illuminate the first photoconductive cell through the surface of the transparent roller, and a second photoconductive cell arranged to receive from the light source light whose intensity varies, in use, only as the intensity of the light source varies, and a bridge circuit including said photoconductive cells, the improvement comprising a variable impedance in one arm of said bridge circuit to adjust the output thereof according to the intensity of light received by said first photoconductive cell, and optical filter means positioned before each of said photoconductive cells, said first and second photoconductive cells having a substantially peak spectral response between 0.5 micron - 0.7 micron and said filter means having a substantially flat spectral response between 0.5 micron - 0.7 micron.
 2. The invention according to claim 1 wherein said variable impedance is a variable resistor.
 3. The invention according to claim 2 in which said variable resistor is in series with another variable resistor which other variable resistor provides fine adjustment of said bridge output.
 4. The invention according to claim 1 wherein said bridge output is connected to electrically operated indicating means calibrated with a linear scale related to print density.
 5. The invention according to claim 1 wherein said bridge output is connected to electrically operated indicating means providing a digital output related to print density.
 6. Apparatus for detecting changes in the thickness of an ink film on a roller system of a printing press comprising in combination: a. A monitoring head including first photoconductive cell means; a transparent roller containing said first photoconductive cell means and provided for rotation with a roller of said printing press; second photoconductive cell means; said first and second photoconductive cell means having a substantially peak response of 0.5 micron - 0.7 micron; an electrical light source disposed between said first and second photoconductive cell means (and capable of emitting light of substantially all colors); first and second optical filter means with a substantially flat spectral response of from 0.5 micron - 0.7 micron and disposed before said first and second photoconductive cell means respectively; and housing means for housing said transparent roller, said second photoconductive cell means, said optical filter means, and said electrical light source; b. A bridge circuit including said first and second photoconductive cell means in first and second arms thereof; impedance means in respective third and fourth arms thereof; said impedance means in said third arm including two variable resistors, one for fine adjustment of the output of said bridge circuit, and the other for adjustment of said bridge output according to the light received by said first photoconductive cell means; responsive means connected To said bridge output; and terminals for connection to energization means for energizing said bridge circuit.
 7. Apparatus according to claim 6 wherein said responsive means is an indicator calibrated with a linear scale related to print density.
 8. Apparatus according to claim 7 wherein said indicator is provided with a digital read-out.
 9. Apparatus according to claim 6 wherein said responsive means is a servo-control system connected to said bridge output for controlling the supply of ink to said printing press.
 10. Apparatus according to claim 9 wherein a plurality of said monitoring heads each connected to respective ones of a plurality of said bridge circuits are provided to monitor differently colored ink films on the rollers of a color printing press, each respectively monitored roller being supplied with ink from controllable supply means, and wherein said variable resistor for varying the sensitivity of said bridge circuit is a stepped resistor for compensating said bridge output for different colored ink films, individual ones of said ink supply means being controllable by corresponding outputs from said bridge circuits.
 11. Apparatus as claimed in claim 9 wherein a plurality of said monitoring heads each connected to respective ones of a plurality of said bridge circuits, the monitoring heads being provided to monitor ink film thicknesses detected at different points across the length of a duct or fountain roller on the inking system of a printing press, said duct or fountain roller being supplied with ink from a duct or fountain including a plurality of adjustable ink duct or fountain keys, individual ones of said keys being controllable by corresponding outputs from said bridge circuits.
 12. Apparatus according to claim 9 including a plurality of said monitoring heads, said monitoring heads being selectively individually connectable to said bridge circuit.
 13. Apparatus according to claim 6 wherein said responsive means includes a warning device for warning a printer when the print density of prints produced by said printing press falls outside predetermined limits.
 14. An improvement in, or modification of, apparatus for detecting changes in the thickness of an ink film on a roller system of a printing press, said apparatus comprising a transparent roller capable of rotating in contact with one of the rollers of the press inking system to pick up an ink film therefrom; a first photocell disposed within said transparent roller; a light source arranged to illuminate said first photocell through the surface of said transparent roller; and a second photocell arranged to receive from said light source, light whose intensity varies, in use, only as the intensity of said light source varies; means for deriving a signal from the output of said first and second photocells in combination, said signal being selected from the type to provide a measure of the thickness of said ink film adhering to said transparent roller and the type to control the ink flow to said roller system of the printing press, said first and second photocells being so connected in a bridge circuit that, in use, the output of said bridge circuit will vary only to the light transmitted through said transparent roller to said first photocell, the improvement comprising a variable impedance connected in one of the arms of said bridge circuit for adjustment to vary the bridge output in relation to the amount of light received by said second photocell, and a variable impedance connected in series with the output of said bridge circuit for adjusting the sensitivity thereof.
 15. A monitoring head for apparatus for detecting changes in the thickness of an ink film on a roller system of a printing press comprising first photoconductive cell means; a transparent roller containing said first photoconductive cell means and provided for rotation with a roller of said printing press; second photoconductive cell means; said first and second photoconductive cell means having a substantialLy peak spectral response of between 0.5 micron - 0.7 micron; an electrical light source disposed between said first and second photoconductive cell means; first and second optical filter means with a substantially flat spectral response of from 0.5 micron - 0.7 micron disposed before said first and second photoconductive cell means respectively; and housing means for housing said transparent roller, said second photoconductive means, said optical filter means and said electrical light source.
 16. Apparatus for detecting changes in the thickness of an ink film on a roller system of a printing press comprising in combination: a. A monitoring head including first photoconductive cell means; a transparent roller containing said first photoconductive cell means and provided for rotation with a roller of said printing press; second photoconductive cell means; an electrical monochromatic light source disposed between said first and second photoconductive cell means and capable of emitting light of a color complementary to the color of the ink the thickness of which is to be detected, said first and second photoconductive cell means having a spectral response to the light from said monochromatic light source; and housing means for housing said transparent roller, said second photoconductive cell means and said monochromatic light source; b. A bridge circuit including said first and second photoconductive cell means in first and second arms thereof; impedance means in respective third and fourth arms thereof; said impedance means in said third arm including two variable resistors, one for fine adjustment of the output of said bridge circuit, and the other for adjustment of said bridge output according to the light received by said first photoconductive cell means; responsive means connected to said bridge output, variable resistive means connected in series with said responsive means for adjusting the sensitivity of said bridge circuit; and terminals for connection to energization means for energizing said bridge circuit. 